An event script is a stored procedure that is run on a replication process (for global actions) or an article (for specific actions) when the associated event occurs. The event scripts available to your specific replication process are specified in the extension file attached to your model. For example, you can create a global event script which handles connection errors and that will be executed whenever the replication process encounters a SQL error.
Event scripts have no symbol in the diagram, but are listed on the Event Scripts tab of a replication process property sheet.